Cardiac ischemia is a condition whereby heart tissue does not receive adequate amounts of oxygen and is usually caused by a blockage of an artery leading to heart tissue. If sufficiently severe, cardiac ischemia results in an acute myocardial infarction (AMI), also referred to as a heart attack. With AMI, a substantial portion of heart muscle ceases to function because it no longer receives oxygen, usually due to significant blockage of the coronary artery. Generally, AMI occurs when plaque (such as fat, cholesterol, and calcium) builds up and then ruptures in the coronary artery, allowing a blood clot or thrombus to form. Eventually, the blood clot completely blocks the coronary artery and so heart tissue beyond the blockage no longer receives oxygen and the tissue dies. In many cases, an AMI proves fatal because too much tissue is damaged to allow continued functioning of the heart muscle. Indeed, AMI is a leading cause of death here in the United States and worldwide. In other cases, although the AMI itself may not be fatal, it strikes while the victim is engaged in potentially dangerous activities, such as driving vehicles or flying airplanes, and the severe pain and possible loss of consciousness associated with AMI results in fatal accidents. Even if the victim survives the AMI, quality of life may thereafter be severely restricted.
Often AMI is preceded by episodes of cardiac ischemia that are not sufficiently serious to cause actual permanent injury to the heart tissue. Nevertheless, these episodes are often precursors to AMI. Episodes of cardiac ischemia may also trigger certain types of arrhythmias that may prove fatal, particularly ventricular fibrillation (VF) wherein the ventricles of the heart beat chaotically resulting in little or no net flow of blood from the heart to the brain and other organs. Indeed, serious episodes of cardiac ischemia (referred to herein as acute myocardial ischemia) typically result in either a subsequent AMI or VF, often within one to twenty-four four hours, sometimes within only a half an hour or less.
Accordingly, it would be highly desirable to provide a technique for reliably detecting acute myocardial ischemia so that the victim may be warned and medical attention sought. If properly warned, surgical procedures may be implemented to locate and remove the growing arterial blockage or anti-thrombolytic medications may be administered. At the very least, advanced warning would allow the victim to cease activities that might result in a fatal accident. Moreover, in many cases, AMI or VF is triggered by strenuous physical activities and so advanced warning would allow the victim to cease such activities, possibly preventing AMI or VF from occurring.
Many patients at risk of cardiac ischemia have pacemakers, ICDs or other medical devices implanted therein. Accordingly, techniques have been proposed for detecting cardiac ischemia using implanted medical devices. In particular, techniques have been developed for analyzing internal electrocardiogram (IEGM) signals in an effort to detect cardiac ischemia. See, as examples, the following U.S. Pat. Nos. 5,113,869 to Nappholz; 5,135,004 to Adams et al.; 5,199,428 to Obel et al.; 5,203,326 to Collins; 5,313,953 to Yomtov et al; 6,501,983 to Natarajan, et al.; 6,016,443, 6,233,486, 6,256,538, and 6,264,606 to Ekwall; 6,021,350 to Mathson; 6,112,116 and 6,272,379 to Fischell et al; 6,128,526, 6,115,628 and 6,381,493 to Stadler et al; and 6,108,577 to Benser. Most ischemia detection techniques seek to detect ischemia by identifying changes in the ST segment of the IEGM that are manifest during cardiac ischemia. The ST segment represents the portion of the cardiac signal between ventricular depolarization (also referred to as an R-wave to QRS complex) and ventricular repolarization (also referred to as a T-wave). The ST segment usually follows an atrial depolarization (also referred to as a P-wave.) Strictly speaking, P-waves, R-waves and T-waves are features of a surface electrocardiogram (EKG). For convenience, herein, the terms R-wave, T-wave and P-wave are used to refer to the corresponding internal signal component as well.
Problems, however, arise when attempting to detect cardiac ischemia using ST segments. Most pacemakers and ICDs initially route electrical cardiac signals through highpass filters to eliminate direct current (DC) components so that the signals can be more easily and reliably analyzed to detect relatively high frequency components such as P-waves and R-waves. However, ST segments primarily consist of very low-frequency signals. So, to permit ST segments to be analyzed for ischemia detection purposes, the highpass filter must be configured to have a sufficiently low cutoff frequency (typically about 0.1 Hz) to allow the low frequency components of the ST segments to pass through the filter. Unfortunately, highpass filters requiring low cutoff frequencies are not well suited for use within implantable medical devices. In particular, such filters require very large capacitors, which add significantly to the size and weight of the implantable device. In addition, because the ST segment primarily consists of low-frequency signals, techniques based upon an analysis of those segments may not be particularly reliable. Accordingly, it would be desirable to provide techniques for detecting cardiac ischemia that do not exploit the ST segment.
One such technique is set forth in U.S. patent application Ser. No. 10/603,429, of Wang et al., entitled “System And Method For Detecting Cardiac Ischemia Using An Implantable Medical Device”, filed contemporaneously herewith, which assigned to the assignee of rights to the present application and is incorporated by reference herein. Rather than examine the ST segment, the technique of Wang et al. instead examines post-T-wave segments, i.e. that portion of the cardiac signal immediately following the T-wave. Although the technique of Wang et al. is very effective in detecting cardiac ischemia while avoiding problems associated with ST segments, it would also be desirable to provide additional or alternative techniques that do not exploit the ST segment.
An alternative to examining the post T-wave segment is to instead examine the T-wave itself. According to some of the patents cited above, various T-wave-based techniques have been developed that exploit: T-wave inversion; changes in the duration or amplitude of T-waves; changes in the rate of rise/fall of the T-wave; or changes in T-wave uniformity. However, it does not appear that any reliable cardiac ischemia detection techniques have been developed that examine the total energy of the T-wave. In contrast, techniques that merely examine the amplitude of a T-wave do not gain a true measure of T-wave energy. Accordingly, it would be desirable to provide a cardiac ischemia detection technique that is based on the total energy of T-waves and it is to that end that aspects of the invention are primarily directed. Nor does it appear that any cardiac ischemia detection techniques have been developed that examine maximum T-wave slope in combination with total T-wave energy and other aspects of the invention are directed to that end as well.
One concern with T-wave-based ischemia detection techniques is that it can sometimes be difficult to distinguish true T-waves from P-waves appearing within channels used to sense ventricular signals. This problem can occur, for example, when using a unipolar lead mounted in the ventricles to sense ventricular signals and a bipolar lead mounted in the atria to sense atrial signals. The problem can also arise when using a bipolar lead mounted in the atria to sense both ventricular signals and atrial signals, i.e. ventricular signals are sensed by using the bipolar lead in a unipolar mode (i.e. ring to case sensing or tip to case sensing) whereas the atrial signals are sensed by using the same bipolar lead in bipolar mode (i.e. tip to ring sensing). Accordingly, it would also be desirable to provide for improved T-wave detection techniques, either for use with cardiac ischemia detection or for other purposes, and it is to that end that still other aspects of the invention are directed.